Page:Popular Science Monthly Volume 55.djvu/165

 a new cycle of rapid disintegrations in the superincumbent strata follow.

An important point to be noticed, and which is evident from two of the reproduced photographs (Figs. 3 and 4), is that the talus has never reached up so high as to check the disintegration at the mouth of the gorge of the Niagara shale and limestone which form the upper one hundred feet of the face, and which exhibit the maximum amount of enlargement which has taken place. The thickness of the Niagara limestone is here so small that it has not been so important an element in forming the talus as it has been farther up the stream, where it is two or three times as thick. Now, while our original supposition was that one quarter of an inch annually was eroded from the upper two hundred feet, this would involve the erosion of a half inch per annum over the top of the gorge to bring the calculation within the limit of ten thousand years. It certainly is difficult for one who examines the facts upon the ground to believe

 —Section, drawn to equal vertical and horizontal scale, showing enlargement of Niagara gorge on the east side at its mouth at Lewiston: 1, Niagara limestone, 20 to 30 feet; 2, Niagara shale, 70 feet; 3, Clinton limestone, 20 to 30 feet; 4, Clinton and Medina shale, 70 feet; 5, Quartzose Medina sandstone, 20 to 30 feet; 6, softer Medina sandstone, 120 feet above water level.

that the crumbling away of this exposed Niagara shale could have been at any less rate than that; so that the estimate of about ten thousand years for the date of that stage of the Glacial period in which Niagara River first began its work of erosion at Lewiston (an